1. Field of the Invention
The present invention relates to a DC brushless motor, a magnetic bearing device and a turbomolecular pump device, in which an rpm can be detected by a simple structure.
2. Description of the Related Art
In many cases in a turbomolecular pump device which is used as a vacuum device such as a semiconductor production line, particularly in a magnetic bearing type turbomolecular pump device that is to be rotated at a high speed of 10,000 rpm or more, DC brushless motors are used for driving rotors. This is because the DC brushless motor is of a compact and high power type and of an energy saving type in comparison with an induction motor, and it is possible to touch down the DC brushless motor after the motor rotates at a low rpm by using the motor as a power generator during the power failure or the like.
However, although the DC brushless motor is provided with a drive circuit for detecting a commutation timing and controlling the rpm, if the drive circuit is broken down so that the motor rotates in an overspeed condition with the rpm exceeding a rated value (for example, 48,000 rpm), there is a fear that the rotor would not be durable against a centrifugal force, resulting in fracture.
In order to prevent such an overspeed, in general, there is provided a protection function for stopping the motor during the overspeed by detecting an output frequency of the motor, i.e., a frequency of the rotational magnetic field.
Also, as a double safety countermeasure in case of the failure of such a protection function, there is provided an rpm detection mechanism for stopping the motor during the overspeed by detecting the rpm of the motor with another independent system.
FIG. 6 shows an overview of such a conventional turbomolecular pump device and a mounting position of a rpm detection mechanism. FIG. 7 schematically shows a structure (a) of the rpm detection mechanism and a detection signal (b).
AS shown in FIG. 6, the conventional rpm detection mechanism in the turbomolecular pump device is provided with an rpm detecting coil 201 and a retainer ring 202. The retainer ring 202 is mounted on a lower end portion of a rotor shaft R. The rpm detecting coil 201 is arranged and fixed on the lower side at a predetermined interval to the retainer ring 202.
As shown in FIG. 7(a), a small size magnetic member is mounted as a target 203 on a lower surface of the retainer ring 202.
In the thus constructed rpm detection mechanism, when the rotor shaft R rotates, the target 203 mounted on the retainer ring 202 rotates across and above the rpm detecting coil 201. Thus, as shown in FIG. 7(b), an induction voltage is outputted in response to the rpm of the rotor shaft R from the rpm detecting coil 201. The rpm of the DC brushless motor is detected from the induction voltage.
Since the conventional device provided with the DC brushless motor and the rpm detection mechanism such as a magnetic bearing device or a turbomolecular pump device requires elongate components for the rpm detection mechanism, the cost for the components increases and the rpm detection mechanism becomes complicated. Therefore, the assembling and adjusting operations are time-consuming.
Also, since the number of the components for the rpm detection mechanism is large, the dimension of the overall device is large and the device is heavy in weight.
In particular in the rotor that rotates at a high speed as in the turbomolecular pump device, since the number of the components is large, its rotary shaft is long and heavy in weight so that the natural bending frequency of the shaft is low. In this case, even if the rpm of the rotor is increased in order to enhance the vacuum performance, when the rpm would be close to the natural frequency of the shaft, the shaft could not rotate due to the resonance. Thus, there is a problem that the rpm could not be increased as desired.